A number of devices are known for heating elements and materials. For example, one known design includes an ohmic heating element mounted for rotation and comprising slip rings for supplying the power required for heating; the material in tape form to be heated is wrapped around the heating element. The temperature is sensed by means such as a temperature sensor firmly coupled with the element to be heated, with the test voltage transmitted through additional slip ring assemblies. One disadvantage of apparatus of this kind is that the slip rings give rise to retarding torques, transfer resistances and thermo voltages. Also, the slip rings are subject to wear. The temperature sensor is coupled firmly with the element to be heated and detects the temperature only at the point where it is located. In addition, the time constant of the sensor reduces the response rate of the arrangement in its entirety.
Also, it has been known to couple temperature sensors to a heating element by a non-stationary or sliding type of engagement. Arrangement of this nature are disadvantageous in that the sensor detects only the temperature prevailing at its instantaneous location. Also, the thermal transfer resistance between the sensor and the test object is difficult to predict, with fluctuations in thermal coupling causing undesirable temperature drift. 1n addition, this arrangement of the temperature sensor causes its time constant to vary so that the rate of response of the control loop will be decreased accordingly.
Further, it has been known to determine the temperature of test objects by radiation measurement. This technique is advantageous in that it operates by means of a pyrometer and does not rely on physical contact between the sensor and the test object. As in the prior teachings discussed above, the temperature value it yields relates only to the partial area of the test object which is sensed by the pyrometer at any one time. Also, the radiation emitted by the test object and received by the pyrometer is affected rather strongly by the nature of the surface of the test object, resulting in test errors in case the surface undergoes changes in the course of the measuring procedure.